Silencing of WIPK and SIPK Mitogen-Activated Protein Kinases Reduces Tobacco mosaic virus Accumulation But Permits Systemic Viral Movement in Tobacco Possessing the N Resistance Gene

Infection of tobacco cultivars possessing the N resistance gene with Tobacco mosaic virus (TMV) results in confinement of the virus by necrotic lesions at the infection site. Although the mitogen-activated protein kinases WIPK and SIPK have been implicated in TMV resistance, evidence linking them directly to disease resistance is, as yet, insufficient. Viral multiplication was reduced slightly in WIPK- or SIPK-silenced plants but substantially in WIPK/SIPK-silenced plants, and was correlated with an increase in salicylic acid (SA) and a decrease in jasmonic acid (JA). Silencing of WIPK and SIPK in a tobacco cultivar lacking the N gene did not inhibit viral accumulation. The reduction in viral accumulation was attenuated by expressing a gene for an SA-degrading enzyme or by exogenously applying JA. Inoculation of lower leaves resulted in the systemic spread of TMV and formation of necrotic lesions in uninoculated upper leaves. These results suggested that WIPK and SIPK function to negatively regulate local resistance to TMV accumulation, partially through modulating accumulation of SA and JA in an N-dependent manner, but positively regulate systemic resistance.